Bolometers are widely used for sensing low radiation of light, generally in the IR band. In most conventional cases, the bolometers are provided in a form of a focal plan array (FPA), wherein the array comprises a plurality of individual sensing elements (hereinafter also referred to as “pixels” or “pixel detectors”). A significant advantage of the bolometer type sensors is their reduced weight and power consumption, particularly due to the fact that they do not require cryogenic cooling. In addition, they are generally much less expensive in comparison with cooled focal plan arrays. However, the typical sensitivity of bolometer type sensors is significantly lower than of cooled-type sensors. Moreover, as bolometer type sensors are very sensitive to temperature variation, they require special means for stabilizing the temperature of the array (FPA) substrate, and for compensating each individual bolometer for said temperature variations. It should be noted that the case that accommodates the FPA contributes roughly 80% of the IR flux for F/1 optics. Thus, it is of vital importance to monitor the case temperature or its radiation.
Vox (Vanadium Oxide) resistors are widely used in typical bolometers, as the Vox has a relatively large TCR (temperature coefficient of resistance), and low 1/f noise contribution.
Typical bolometer FPAs are required to detect radiation with a resolution in the order of 50° mK of the scenery temperature. The temperature variations at the bolometer due to the heat variations within the scenery are in the order of 0.01-0.1° mK. It should be noted that in order to sample those temperature variations, it is required to heat the active resistor (the resistor which is exposed to the scenery) of the bolometer by a temperature in the order of few degrees. Said necessity to provide a sensitivity and resolution in the range of at least 40 orders less than the heating of the active bolometer resistor enforces the use of a differential measurement. The most common and simple circuitry that applies differential measurement is the Wheatstone bridge, and a circuitry which includes Wheatstone bridge is indeed commonly used in bolometer-type FPAs.
However, even though a Wheatstone bridge which performs a differential measurement is applied, the uncooled bolometer-type FPAs mentioned in the prior art are still very sensitive to variations in the ambient temperature, and special compensation circuitry is required for compensating in the FPA pixel level. More particularly, special circuitry is required to compensate for the non-uniformity of the detectors (i.e., to compensate for their different offset and gain), and to further compensate for the non-uniform effect of the change of the case temperature on each detector. The said latter non-uniformity arises from the fact that each detector has a different relative location with respect to the case walls.
In order to account for the non-uniformity of the FPA pixel detectors, prior art bolometer-type FPA manufacturers, or the users themselves commonly perform pre-measurements which determine the gain and offset of each pixel detector. The measurements are performed for constant, predefined ambient (case) and substrate temperatures. The results of the measurements are provided in two matrices (or look up tables), a gain non-uniformity matrix, and an offset non-uniformity matrix. More particularly, by using said two matrices the gain and offset of each pixel detector are adjusted during the actual use of the FPA. It should be noted that the offset matrix is also updated periodically (for example, every 2-3 minutes) at times when a shutter is closed and masks the FPA from scenery radiation. Of course, the FPA cannot be used during the times in which the shutter is closed and the offset matrix update is performed. Said procedure of correction is generally referred to as NUC (Non-Uniformity Correction).
As said, variations in the ambient (case) temperature affect differently each pixel detector, according to the difference of exposure of each pixel detector to the radiation from the case walls. For example, the detectors that are close to the edges of the FPA, and therefore closer to the case walls are more vulnerable to variations in the case temperature than those that are located at the center of the FPA (and therefore farther from the case walls). Therefore, in order to account for said non-uniformity of exposure to the case walls, prior art bolometer-type FPA manufacturers or the users themselves perform also pre-measurement of the response of each pixel detector to variations in the ambient (case) temperature. These latter measurements depict the readout variation of each pixel detector to different case temperatures (while the scenery radiation and the substrate temperature are kept constant). Said latter measurements result in a third, case temperature matrix. During the actual use of the FPA, an additional compensation circuitry performs actual measurement of the case temperature, and using said case temperature matrix, the circuitry provides further compensation in the pixel level to the readout in order to correct its variations evolving from changes in the ambient (case) temperature.
The measurement of the case temperature during said tests for obtaining said third matrix, as well as the actual measurement of the temperature case in order to compensate for the temperature variations during the actual use of the FPA, use temperature sensors that are externally attached to the case. However, the numerous FPA pixel detectors are sensitive to variations of the IR radiation from the case walls, a radiation which is only indirectly correlated to the variations of the case temperature (as measured by said external temperature sensors). Said indirect correlation results in inaccuracies in the compensation that the compensation circuitry provides. Some prior art circuitries apply additional transformation means for transforming said temperature matrix data to a radiation data matrix in order to account for the said indirection of measurement.
It is an object of the present invention to provide means for performing direct pre-measurements and direct pre-determinations of the effects of various levels of IR case radiation on each pixel detector of the FPA.
It is another object of the present invention to provide means for directly measuring the radiation within the FPA case.
It is still another object of the present invention to provide circuitry means for using said radiation measurement, together with said pre-measurement results to compensate each readout value of each pixel detector for the effects due to case radiations.
Other objects and advantages of the present invention will become apparent as the description proceeds.